1. Field of the Invention
This invention relates to an inhaler whereby a drug in the form of a powder may be delivered to a patient.
2. Description of the Related Art
Inhalers are known which operate solely by the patient breathing in. The resultant inspired air carries the powder incorporating a medicament or drug into the lungs of the patient. An example of such an inhaler is described in EP-A-539469. The inhaler described in this patent specification comprises a body defining a storage chamber for the substance to be delivered and further defining an inhalation passage through which air is drawn via a mouthpiece. A metering member operates to transfer a volumetric dose of the substance from the storage chamber to the inhalation passage. The metering member is provided with dispensing cups and is moveable between a first position in which a dispensing cup is presented to the storage chamber to receive a dose of the substance to a second position in which a dose of the substance is presented to the inhalation passage.
Other dry powder inhalers are described in EP-A-0079478, EP-A-0166294, GB-A2165159, U.S. Pat. No. 2,587,215, U.S. Pat. No. 4,274,403 and EP-A-0069715.
The powder which is located in such dry powder inhalers may typically be a mixture of the medicament itself and a material such as lactose, the whole being in the form of a micronised powder. The presence of the lactose assists the free flow of the drug which might otherwise tend to agglomerate or adhere to the internal surfaces of the inhaler.
The formulation of the powder mix and the size of each dose are carefully controlled so that the desired amount of the powder, and therefore of the medicament, is of a size to reach the required area of the lungs. This amount is known as the respirable fraction. However, in use, a portion of the dose does not reach the lungs but remains in the throat of the patient. For some medicaments, this is undesirable and there is a in the throat of the patient. For some medicaments, this is undesirable and there is a need to provide an inhaler which is capable, in use, of retaining powder with a particle size above that of the respirable fraction rather than allowing such material to enter the patient""s mouth.
There have already been proposals to deal with inhaler powders containing particles of relatively large size. For instance, WO-A-92/05825 describes a dry powder inhaler which includes a dispensing chamber into which the powder is introduced in operation. The dispensing chamber is connected via a duct to a mouthpiece through which the patient may inspire so as to cause air to flow through the chamber, the duct and then the mouthpiece. Within the duct there is located a stationary plate substantially transverse to the flow direction through the duct. As a result, substantially all the air flow through the duct is deviated abruptly to avoid the plate and the large particles impact with the plate due to their inertia. By such means the larger particles either break up or remain in the inhaler, reducing their delivery to the mouth and throat region of the patient, so reducing side-effects of the drug.
As indicated, devices such as those described in WO-A-92/05825 rely on one or more plates or baffles within the device to break up, or slow the progress of, larger particles. The small particles are able to change direction rapidly with the air flow and therefore pass through the device. A disadvantage of such a device is that its effectiveness depends on the period during which air is drawn through the device and the velocity of the air. There is a need for a device which is effective regardless of the air flow time and the air flow velocity.
It has also been proposed to use cyclonic separation of a larger particles from smaller particles to reduce the number of larger particles reaching the mouthpiece of the inhaler. The larger particles are of course heavier than the smaller particles so that when a mixture of the particles is introduced into a vortex or other cyclonic system the centrifugal forces then acting on the particles tend to throw the heavier particles further away from the centre of rotation of the system than the lighter particles. The use of a cyclonic system for holding back the heavier particles is exemplified by EPA-407028, which describes an inhaler in which an air/powder mixture is introduced into a cylindrical cyclone chamber through a tangentially disposed inlet. Air/powder mixture is withdrawn from the cyclone chamber by way of an outlet which is orthogonal to the inlet and lies on the axis of the cylindrical chamber. This arrangement is intended to select the smaller particles for inhalation. A comparable system is described in GB-A-1478138 which shows a powder inhaler provided with air inlets so arranged as to create a vortex within a powder storage chamber in order to trap heavier particles within the storage chamber while allowing lighter particles to escape into a housing for inhalation; this housing has air inlets adapted to create a vortex within the housing, which serves to throw heavier particles nearer to the wall of the housing than lighter particles so that they can be trapped and held back from leaving the housing.
Both baffle systems and cyclone systems can serve two purposes, firstly to break down agglomerated particles and secondly to trap agglomerates that have not been broken down. However, the design of such system for maximum efficiency in either of these respects is made more difficult by the limited nature of the power of suction that a typical patient is capable of applying to the mouthpiece. This difficulty is especially noticeable in the case of a cyclone system because the efficiency of a cyclone separator is dependent on the speed of circulation of the air/powder mixture, a speed which is largely determined by the velocity with which the air/powder mixture enters the cyclone chamber.
The present invention provides a dry powder inhaler comprising a body defining a storage chamber for a powder and further defining an inhalation passage through which air is drawn via a mouthpiece, the mouthpiece having an inlet and an outlet and defining a flow duct for the flow of an air/powder mixture therethrough, the flow duct extending between said inlet and said outlet and including a circulatory section (for effecting centrifugal separation of heavier and lighter particles) in which the flow duct is in the form of one or more passageways, said one or more passageways being circulatory about an axis extending between said inlet and said outlet, and the flow duct further including a cyclone chamber (for restraining heavier particles from reaching said outlet) between said circulatory section and said outlet.
The circulatory section of the flow duct serves primarily for effecting separation of heavier particles from lighter particles but also serves to convert the linear flow of the air/powder mixture entering the mouthpiece into a circulating flow in a smooth manner, with relatively little loss of kinetic energy, so that the air/powder mixture enters the cyclone chamber at relatively high speed.
Centrifugal forces act on the particles of the powder as the air/powder mixture circulates about the axis of the mouthpiece through the passageway(s) of the circulatory section. These forces set up a size distribution of particles across the passageway(s), the heavier particles being at the outside of the passageways, away from the axis of the mouthpiece, and the lighter particles at the inside of the passageway(s), towards the axis of the mouthpiece. Therefore, the powder enters the cyclone chamber with the lighter particles so positioned as to be preferentially drawn towards the centre of the chamber, from which air is being removed due to suction applied to the outlet of the mouthpiece by the patient. The heavier particles, on the other hand, remain in orbit in the chamber until the patient ceases to apply suction force.
In effect, the flow duct divides into two paths after the circulatory section. One path, followed by the lighter particles, leads to the outlet of the mouthpiece and to the patient, and the other path, followed by the heavier particles, leads to the radially outer region of the cyclone chamber. This maximises the proportion of lighter particles delivered to the patient, while minimising the proportion of heavier particles inhaled.
Earlier proposals to make use of cyclone chambers in powder inhalers have been for the purpose of effecting de-agglomeration, or for separating from one another the heavier and lighter particles of a powder in an air/powder mixture in which the particles have not previously been subjected to any separation and are still substantially uniformly distributed. In this invention the primary function of the cyclone chamber is not to effect de-agglomeration and/or separation but to retain in orbit the heavier particles which have previously been separated from the lighter particles by the circulatory section, thereby retaining them in the mouthpiece, to be discarded after the inhaler has been used. Some de-agglomeration of particles and some separation of particles may take place in the cyclone chamber but this is incidental and not essential. Accordingly, the term xe2x80x9ccyclone chamberxe2x80x9d is used herein to mean any chamber within which an air/powder mixture can be caused to circulate with an angular velocity generating centrifugal forces sufficient to maintain heavier particles of a powder mixture in orbit about an axis through the chamber.
The circulatory section of the flow duct may have any conformation adapted to effect circulatory flow of the air/powder mixture about the axis of the mouthpiece in such manner that the heavier and lighter particles of the powder are separated from one another by centrifugal forces.
The circulatory section is preferably a helical section in which the flow duct is in the form of one or more passageways of substantially helical conformation, preferably arranged as a helix or as helices about the aforementioned axis. Preferably the diameter of the or each helix increases in the direction from the inlet towards the outlet. The terms xe2x80x9chelixxe2x80x9d and xe2x80x9chelicalxe2x80x9d are used to include both spiral and helical conformations, flat or extended, and with constant or variable diameters, and to include less than one turn and one or more turns.
Preferably, the cross-sectional area of each of the passageways in the circulatory section decreases in a direction from the inlet towards the outlet.
Preferably, the flow duct includes a section, located between the inlet and the helical section which is arranged so that the air/powder mixture flowing through the mouthpiece makes an abrupt change of direction. Accordingly the air/powder mixture encounters, in this section, a surface against which it impacts. This action causes agglomerated particles to break up.
Preferably the helical section of the flow duct terminates short of the outlet and the flow passage between said helical section and the outlet includes a section of cross-sectional area substantially less than the pitch circular diameter of the or each passage within the helical section. More preferably, said outlet section is located along the longitudinal axis of the helical section.
Preferably, the cyclone chamber extends, transversely of, for example at right angle to, the axis between the inlet and the outlet, the centre of the chamber preferably being on that axis.
Preferably, the flow duct is such that air passing from the helical section to the centrally located outlet section creates a Venturi effect producing a negative pressure in the outer chamber which serves to retain particles which have deposited in the chamber.
The present invention also provides a mouthpiece for a dry powder inhaler, the mouthpiece being for attachment to the body of the inhaler, and having an inlet and an outlet and defining a flow duct for the flow of an air/powder mixture therethrough, the flow duct extending between said inlet and said outlet and including a circulatory section in which the flow duct is in the form of one or more passageways, said one or more passageways being circulatory about an axis extending between said inlet and said outlet, and the flow duct further including a cyclone chamber between said circulatory section and said outlet.